1. Field of the Invention
The present invention relates to a coupling method of an optical fiber coupler, and particularly to a coupling method including (1) making a pair of optical fibers with different photosensitivity fused and drawn, (2) irradiating the pair of optical fibers with intense ultraviolet light (hereafter referred to as UV radiation) at the same time, (3) supervising and gradually adjusting the UV radiation to obtain asymmetric refractive index of the pair of optical fibers for decreasing maximum coupling ratio, thereby achieving an optical fiber coupler with optimum flat spectrum.
2. Related Art
Optical fiber couplers are popularly used in optical fiber transmission systems and optical fiber detection systems. It is well known that an optical fiber coupler is fabricated by keeping cores of a pair of optical fibers close with each other, as shown in U.S. Pat. Nos. 4,763,272, 5,410,626 etc.
The coupling method of keeping cores of a pair of optical fibers close with each other can easily adjust coupling ratio. As disclosed in U.S. Pat. Nos. 4,763,272, 5,410,626, one of the optical fibers is pre-pulled for being tapered, and then it is fused and drawn together with another optical fiber. Geometric asymmetry of the two optical fibers reduces maximum coupling ratio so as to make coupling spectrum flat. However, stepping motors drive the optical fiber to be tapered, hence neither precision of pulling nor flatness of coupling spectrum is controllable. Moreover, the coupling spectrum is determined after fusing and drawing the two optical fibers, so it is not adjustable during coupler fabricating process.
Accordingly, an object of the present invention is to provide a coupling method of an optical fiber coupler, which includes supervising and gradually adjusting light energy loss spectrum so as to achieve maximum flat coupling spectrum, thereby improving fabricating process and reducing deficient products.
The coupling method includes the steps: (1) making a pair of optical fibers with different photosensitivity fused and drawn, (2) irradiating specific portions of the optical fibers with UV radiation at the same time to make refractive index of the optical fibers markedly different, (3) supervising light energy input loss spectrum and gradually adjusting UV radiation so as to decrease maximum coupling ratio, thereby achieving optimum flat spectrum.
The pair of optical fibers comprises a single-mode optical fiber of poor photosensitivity and a boron-germanium codoped optical fiber of excellent photosensitivity.